The Lunar Prospector Alpha Particle Spectrometer (LP APS)
searched for lunar surface gas release events and mapped
their distribution by detecting alpha particles produced by
the decay of gaseous radon-222 (5.5 MeV, 3.8 day half-life),
solid polonium-218 (6.0 MeV, 3 minute half-life), and solid
polonium-210 (5.3 MeV, 138 day half-life, but held up in
production by the 21 year half-life of lead-210). These
three nuclides are radioactive daughters from the decay of
uranium-238. Radon reaches the lunar surface either at areas
of high soil porosity or where fissures release the trapped
gases in which radon is entrained.

We have examined APS data within ± 45 degrees of the
equator acquired during periods of low interplanetary alpha
particle flux. The spectra were summed over all LP mapping
cycles when the instrument was turned on (approximately 229
days over 16 months). To yield lunar alpha particle maps, we
summed over a 0.2 MeV energy range centered on each of the
three alpha particle energies noted above. The LP APS found
only a faint indication of alpha particles resulting from
the decay of polonium-218 and only a marginal detection of
alpha particles from polonium-210. However, our radon-222
alpha particle map indicates that radon gas is presently
emanating from the vicinity of craters Aristarchus and
Kepler. The LP gamma-ray spectrometer, which effectively has
significantly higher spatial resolution than the APS,
identified thorium enrichments at these two craters. Thorium
and uranium are both incompatible elements whose lunar
surface abundances are highly correlated; thus, it is likely
that the radon-222 alpha particles measured using the LP APS
originate from Kepler and Aristarchus. Our detection of
radon over Aristarchus is consistent with the results of the
Apollo 15 APS.

The author(s) of this abstract have provided an email address
for comments about the abstract:
stefs@lanl.gov